We sold our long term home in June 2018 and took the money and started to build our new home right away. Is this not the same as rolling over capital gains from our home to another?
No. The system of deferring gain on the sale of a primary residence was repealed in 1997. The law now allows the exclusion of gain rather than the deferral of gain. See Home Sale Exclusion of Gain for more information about the rules that apply to selling your home.
Each transaction is treated separately.
If you lived in your previous home for at least 2 years of the last 5 years and owned it for at least those two years, you can exclude the first $250,000 of gain (or $500,000 if married filing jointly) and only pay capital gains tax on the gain over the exclusion limit. (Additional special rules apply if you ever used the home as a rental or took the home office deduction.)